Porcelain enamel



. 1 This invention relat es to Patented Sept. 6, 1949 T UN [TED TAT'ES PATENT H 2,481,474

PORCELAIN ENAMEL Gilbert Goodman, Bayonne, N. J., assignor to B ell Telephone Laboratories, Incorporated, New

York, T a corporation of New York I: Application April 1946,

Serial N0. 666,509

m omposi i and methods of making them and more particularly to ceramici'coatinglcompositions having a high ,adherence-to-metal characteristic and a .-minimum of vesicular nature and to methods of making'them.

In the manufacture of wire-wound resistors utilizing nickel-chromium resistance wire wound on ceramic cores, it has hitherto been proposed to insulate the wound resistance wire with a ceramic coating prepared from a thixotropic enamel slip which may include a frit, and ingredients such as colloidal clay, an opacifier and magnesium carbonate. After this composition had been fired, it was discovered that the coating adhered to the ceramic core but did not adhere strongly to the nickel-iron alloy terminal bands, thereby weakening and lessening the effectiveness of the coating. It was also found that the fired enamel had a highly vesicular structure due to the inability of the evolved carbon dioxide to escape because of the viscous nature of the enamel in the molten condition, and in part probably due to the decomposition products of the colloidal clay. If the alkalinity .of the frit is sufiicient to cause defiocculation, the magnesium carbonate may be omitted and a less vesicular end product will be obtained. However, the structure is still vesicular and the adherence-to-metal qualities are not improved by the omission of the magnesium carbonate.

The objects of this invention are to provide improved ceramic coating compositions and methods of producing them.

In accordance with one embodiment of this invention, strontium chloride or barium chloride is added to an enamel slip to give the enamel a high adherence-to-metal characteristic and a dense structure after firing.

It has been found that a suitable frit may be made up according to any of the formulae given below:

Ingredient Parts by Weight Flint (SiOz) 27 34. 7 85. 7 36.9 Red Lead (P1330!) Boric Acid (B31303) 10 8.9 30. 9 29. 9 Borax (NezB4O7.10Hz0). 26 30.8 6.6 Potassium Nitrate (KNOa) 5. 7 5. 8 Chronn'c Oxide (Cr203) 0.9 0.9 9 Cobalt Oxide (00300.... 5 0. 5 0. 5 0. 8 Sodium Nitrate (N aN a).. 0

Manganese Dioxide (M1102 6 Titanium Dioxide (TiOz) 2.

Potassium Chloride (K01).-. 4. 6

Sodium Fluoride (N aF) 1. 6

The mixed frit composition is then heated and when molten, it is allowed to flow into relatively cold water, thus shattering" the frit. The frit 3 Claims. (c1, 106-49) is further pulverized matau iii'iirdi btfieftit- .able pulveriz'ing. device.

The v rer d i I. P enamel slip which may have "theioll portions by weight:

Frit 'j Bentonite (colloidalclay) 2.0 O-pacifier (zirconium oxide) 9.0 Strontium chloride 2.0

These components may be mixed in a ball mill with a suitable amount of Water to provide a fluidity and viscosity consistent with the planned use of the enamel and with the available apparatus. A suitable defiocculant may be added to the slip if desired.

Departures in the proportions shown in the examples may be made to change the maturing temperature of the enamel or the degree of water solubility of the frit or both. Barium chloride may be used in place of strontium chloride; however, the latter is preferable because of its lower melting point.

Chromic oxide acts as a pigment and also to help prevent the dissolving of the Nichrome resistance wire during the firing. Cobalt oxide acts as a pigment and to promote adherence of the enamel to metals. The .opacifier, which may be zirconium oxide, provides opacity. The bentonite or colloidal clay is added for body. Lead borosilicate ceramic compositions, that are not metal adherent, will be rendered metal adherent by the addition of strontium chloride or barium chloride and those that are to some degree metal adherent will have an improved metal adherence characteristic by the addition of strontium or barium chloride.

An object may be coated by dipping it in the slip or by allowing the slip to flow upon the ob ject, thereby covering the object. The covered object is then dried in a normal atmosphere and subjected to heat to fire the enamel. This proccess may be repeated to increase the thickness of the coating. It will be found that after firing the enamel, the structure is relatively dense and any metal covered by the enamel will be tenaciously adhered to by the enamel.

Strontium or barium chloride is effective in eliminating the vesicular structure of the enamel in concentrations from about 1 per cent to 5 per cent by weight. Contrary to expectations, the addition of strontium chloride to the enamel while it does improve the melting properties of the enamel, decreases rather than increases the fluidity, that is, strontium chloride although nora suitable substance to deflocculate the enamel;

When a sufficient amount of strontium chloride necessary to inhibit the formation of a vesicular enamel structure was added, it was found that the desired adherence-to-metal was also attained. 7

What is claimed is:

1. A lead borosilicate enamel slip comprisinga lead borosilicate frit, colloidal clay, and in addition to said frit and clay between about 1 per "cent and about 5 per cent by weight of a compound selected from the group consisting of strontium chloride and'barium chloride.

2. A lead borosilicate enamel slip comprisin a lead borosilicate irit, colloidal clay and an aqueous solution of strontium chloride, said strontium chloride being present in an amount between part of cobalt oxide.

1s characteristm a Number Number about 1 per cent and about 5 per cent by weight of the slip.

3. A ceramic slip composition comprising by weight 2 parts of colloidal clay, 9 parts of zirconium oxide, 2. quantity of strontium chloride varying from 1 per cent to 5 per cent by weight of the whole of .said ceramic composition, and 100 parts oi .frit, said frit comprisingby weight 34.7 parts'of flint, 18.5 partso'f red"lead,'8.9 parts of boric acid, 30.8 parts of borax, 5.7 parts of potassium nitrate, 0.9 part of chromic oxide and 0.5

GILBERT GOODMAN.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Name Date Migeat Nov. 24, .1931 Fe n at a 1 8 4 FOREIGN PATENTS Country; Date Germany i 1934 

